Is the Complement Activation Product a Proinflammatory Molecule? Re-evaluating the Evidence and the Myth

This information is current as Liam G. Coulthard and Trent M. Woodruff of September 23, 2021. J Immunol 2015; 194:3542-3548; ; doi: 10.4049/jimmunol.1403068 http://www.jimmunol.org/content/194/8/3542 Downloaded from

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Is the Complement Activation Product C3a a Proinflammatory Molecule? Re-evaluating the Evidence and the Myth Liam G. Coulthard and Trent M. Woodruff The complement activation product C3a is often de- which this agent was used (3). We wish to use this review to scribed as a proinflammatory mediator, alongside its draw much-needed attention to the potential anti-inflammatory downstream cousin, C5a. However, emerging studies rolesofC3a,soastorefocusthediscussiononitsrevisedpo- show that C3a has several anti-inflammatory facets in sition in the immune response, allowing new strategies for vivo. For example, in the acute inflammatory response, therapeutic targeting of C3a. Although the modern literature surrounding C3a and C5a C3a acts in direct opposition to C5a, through prevent- Downloaded from ing the accumulation of neutrophils in inflamed tissues frequently describe divergent functions for these peptides, their by independently regulating their mobilization. This discovery and characterization may explain why they are so acute, protective, and opposing activity of C3a to C5a often portrayed together as proinflammatory mediators. In the is also illustrated in models of septicemia. In this article, late 19th century, it was shown that Ag-exposed serum con- we reinvestigate the discovery and original classification tained both a heat-labile and a heat-stable bacteriocidal ele- ment. The heat-stable element was demonstrated to be Ab, of C3a as a proinflammatory mediator and highlight the http://www.jimmunol.org/ whereas the heat-labile element was termed “complement” by emerging studies demonstrating anti-inflammatory Paul Ehrlich, as it complemented the Ab response. The ana- effects for C3a in the immune response. It is our hope phylatoxins were discovered through incubation of serum that this review illuminates these apparently contradic- with Ag–Ab complexes, thus activating the classical pathway, tory roles for C3a and challenges the general dogma sur- and were named for their histamine-releasing ability (4). Al- rounding C3a, which, historically, has ubiquitously though unconfirmed at the time, the origin of the anaphyla- been described as a proinflammatory mediator. In light toxin activity was suspected to be complement, because of of this, we urge investigators to use “inflammatory mod- its heat-labile nature (5). This activity was finally ascribed to ulator” as the descriptor for C3a. The Journal of Im- complement split products with the identification of multiple by guest on September 23, 2021 munology, 2015, 194: 3542–3548. serum components of complement and the purification of C3a and C5a in the late 1960s. Initial experiments demon- strated that the release of the elusive was de- he complement cascade is a key component of the pendent on the presence of the initiating factors of the com- immune system and is essential in providing a bal- plement cascade and that the anaphylatoxin was released at, or anced response to injury and infection. Complement T shortly after, the involvement of C3 (6). With purification, can be activated through multiple initiating pathways, but all both C3a and C5a were shown to induce smooth muscle routes lead to the cleavage of the central complement com- contraction through release of histamine in a mutually re- ponents C3 and C5 to generate the small bioactive fragments dundant manner, both having similar “activity” at the guinea C3a and C5a (1). Both C3a and C5a are frequently and pig ileum (7). This led to the adoption of the term “com- consistently described in the literature as proinflammatory plement anaphylatoxin” for these peptides, although this label mediators, despite a limited number of reports of true pro- has recently been challenged (8) because it does not truly inflammatory activity for C3a in vivo. Furthermore, many encompass their widespread functions. reports, in fact, demonstrate an anti-inflammatory role for this major complement cleavage product in infection and dis- ease. To add to the confusion, the only reported in vivo C3aR C3a generation, structure, and signaling antagonist (SB290157), which is used frequently in research Cleavage of C3 results in the formation of two split products: studies, was, in fact, demonstrated to act as a full agonist (2), C3a and . C3a is generated at the convergence of all known thus clouding interpretations of results in disease models in complement-activation pathways, and its sibling at the point of

School of Biomedical Sciences, University of Queensland, St. Lucia 4072, Queensland, Abbreviations used in this article: IR, ischemia-reperfusion; RAGE, receptor for ad- Australia vanced glycosylation end-products; Treg, regulatory T cell. ORCID: 0000-0003-1382-911X (T.M.W.). Copyright Ó 2015 by The American Association of Immunologists, Inc. 0022-1767/15/$25.00 Received for publication December 9, 2014. Accepted for publication February 12, 2015. Address correspondence and reprint requests to Dr. Trent Woodruff, School of Bio- medical Sciences, University of Queensland, St. Lucia 4072, QLD, Australia. E-mail address: [email protected]

www.jimmunol.org/cgi/doi/10.4049/jimmunol.1403068 The Journal of Immunology 3543 cleavage, C3b, continues the propagation of the complement tor phosphorylation (23), are inhibited by the presence of C5a cascade (9). Classically, cleavage of C3 results from the action in human granulocytes (24). Studies in this area were perhaps of one of the C3 convertases, which are multiprotein com- the first reports of functional antagonism between C3a and plexes formed from the upstream cleavage events of comple- C5a at the molecular level. Until recently, C3a was thought to ment activation. Additionally, C3 can be cleaved by multiple be the only ligand for C3aR; however, the neuropeptide serine proteases present at sites of inflammation, most notably TLQP-21, a cleavage fragment of the VGL propeptide, was blood proteases, such as thrombin, and immune cell–derived shown to bind and activate murine C3aR through confor- proteases, such as cathepsin (10). mational change of the ligand upon receptor binding (25, 26). Structurally, C3a is a 77-aa peptide consisting of three to Additionally, the actions of C3a need not be mediated solely four helical regions and a series of nonregular residues at the through its canonical receptor, C3aR. In a model of experi- C-terminal responsible for binding at C3aR (11, 12). Short mental autoimmune encephalitis, ectopic CNS C3a expression +/+ portions of the C terminus were demonstrated to be necessary worsened disease outcome in C3ar1 animals. However, in both for agonist activity at C3aR and intrinsic antimicrobial mice lacking C3aR, the induced C3a expression improved activity (12, 13). The activity at C3aR of the terminal residues pathology, suggesting that the actions of C3a in CNS disease in isolation is far lower than that of the native molecule, and may not be confined solely to its interaction with C3aR (27). there is evidence that these flexible C-terminal residues are This finding was corroborated in a second paper from the stabilized in the conformation necessary to bind C3aR by the same group, demonstrating a protective effect for CNS C3a upstream a helix (14). Hence, this C-terminal region of C3a expression in LPS-induced endotoxic shock that was not di- Downloaded from minished in the absence of C3aR (28). has been a nidus for drug-design research, both in terms of C3a was also shown to bind to the receptor for advanced therapies capable of modulating C3aR activity and those that glycosylation end-products (RAGE) with a high affinity, but take advantage of the intrinsic antimicrobial abilities of the this interaction is more complex than a simple ligand–receptor molecule (15, 16). After its generation, C3a is cleaved quickly interaction (29, 30). Ruan et al. (29) demonstrated that C3a at the C-terminal arginine to form C3a-desArg. This molecule is able to form a complex with CpG oligonucleotides to en- http://www.jimmunol.org/ has no detectable activity at C3aR, but it was shown by some hance IFN-a release from mononuclear leukocytes. Intriguingly, groups to bind the second receptor for C5a, C5aR2 (C5L2) there is no demonstrable effect of a C3a–RAGE interaction (17, 18). The interactions with C5aR2 exhibit effects on alone, despite strong and specific binding between the two. metabolism and, in this sphere, C3a-desArg is homologous to acetylation-stimulating protein (19). C3a activity on immune cells However, this review focuses on the interaction of C3a with C3aR is expressed by all leukocytes of myeloid (and several its canonical receptor C3aR. It should be noted though, that nonmyeloid) lineages; however, the functional characteristics there are additional factors emerging in the literature that also

of the receptor, as well as the response to C3a, are dependent by guest on September 23, 2021 intersect with the C3a axis (detailed below and summarized in on cell type. In the past few decades the literature concerning Fig. 1). C3aR is a classical seven-transmembrane, G protein– C3a and its actions at the cellular level demonstrated a bipolar coupled receptor that shares close homology to the receptors role for this molecule in different immune cell types (Fig. 2). for C5a, C5aR1, and C5aR2. C3aR couples to heterotrimeric For example, despite a canonical proinflammatory histamine de- G proteins and demonstrate a promiscuity of G protein in- granulation response for C3a on mast cells (31), our laboratory teraction that is dependent on cell type (20–22). Intriguingly, recently showed a potent anti-inflammatory response for C3a the internalization and recycling of C3aR, which are depen- on neutrophils, by attenuating their mobilization into the cir- dent on G protein–coupled receptor kinase–mediated recep- culation following injury (32). Uninhibited mobilization can

FIGURE 1. Diagrammatic representation of receptor– ligand interactions surrounding C3a. C3a, generated from complement pathway activation, interacts with its canonical receptor, C3aR, and RAGE. Functional in- teraction with RAGE is dependent on formation of a C3a–CpG oligonucleotide (oligo) complex. The VGF cleavage product, TLQP-21, is also a ligand of C3aR. The terminal arginine of C3a is cleaved by serum car- boxypeptidases (CPase) to form C3a-desArg (acetylation stimulating protein). C3a-desArg may bind and signal through C5aR2. 3544 BRIEF REVIEWS: C3a AS A PRO-/ANTI-INFLAMMATORY MOLECULE

FIGURE 2. Functions of C3a on the cell types of the immune system. C3a acts in an anti-inflammatory manner to decrease cytokine release from LPS-primed, nonadherent PBMCs; induce direct antimicrobial ac- tions on pathogens; and inhibit the mobilization of neutrophils from the bone marrow reservoir (left panel). C3a acts in a proinflammatory manner to increase cytokine release from LPS-primed, adherent PBMCs; induce degranulation in eosinophils and mast cells; increase inflammatory mediator production in macro- Downloaded from phages/monocytes; and modulate the T cell response through the suppression of Tregs and induction of a Th1-polarized response (right panel). http://www.jimmunol.org/

lead to an increase in neutrophil numbers at the site of injury, (39), in vitro investigations into the effects of C3a on LPS- which can significantly worsen disease progression. induced cytokine release by PBMCs demonstrate differences in by guest on September 23, 2021 The reports on the function of C3a on neutrophils are action depending on the phenotype of the cell. C3a augmented complicated, owing to past difficulty in obtaining pure neu- the proinflammatory cytokine production of adherent cells but trophil cell isolates. Indeed, many of the earliest reports in- suppressed that of nonadherent cells (35, 36, 41). dicating that C3a induced neutrophil activation were later Interestingly, despite a marked lack of degranulation effect refuted because of contamination with nonneutrophil gran- of C3a on neutrophilic granulocytes, in other granulocytes ulocytes (33). What is now clear is that, despite high ex- C3a induces a degranulation response. For example, in hu- pression of functional cell surface C3aR on neutrophils, C3a man eosinophils, C3a stimulates calcium mobilization, oxidative does not chemoattract or stimulate degranulation of neu- burst, and degranulation (42, 43), whereas in mast cells, C3a can trophils, despite inducing downstream signaling through induce histamine secretion in a dose-related fashion (44). This Erk1/2 and Akt (33, 34). Interestingly, the signaling produced eosinophil and mast cell specificity highlighted C3aR as a drug- by C3a stimulation of neutrophils was shown in one report to target candidate for allergic conditions. Such inferences have 2 2 be dependent on the presence of C5aR2, suggesting that been strengthened through observations that C3ar1 / C5aR2 may contribute to C3a signaling through interactions animals exhibit reduced responses in atopic diseases, such as with C3aR (34). However, our recent studies indicate that, allergic asthma and allergic dermatitis (45), again suggesting at the level of the bone marrow, C3a prevents migration of a proinflammatory role for C3a in these situations. neutrophils into the circulation by acting in direct opposition Administration of C3a also was demonstrated to augment to neutrophil-mobilizing factors, such as G-CSF, in a manner the T cell response, promote T cell proliferation, and prolong reminiscent of stromal cell–derived factor-1 (32). the inflammatory response through suppressing regulatory C3a also can induce signaling in human monocytes and T cell (Treg) production (46, 47). Additionally, T cells gen- monocyte-derived macrophages; however, this interaction, erate intracellular C3a that contributes to their survival (48); 2 2 with TLR-4 costimulation, induces the production of proin- accordingly, T cell populations are reduced in C3ar1 / mice b a flammatory mediators, such as IL-1 , TNF- , IL-6, and PGE2 in several models of disease (46, 47). Although broad T cell (35–39). This suggests that, in the chronic phase of inflam- expression of C3aR remains controversial (49), TCR stimu- mation, where monocyte/macrophage responses become more lation upregulates C3aR mRNA expression in isolated T cell predominant over neutrophils, C3a may indeed act as a clas- populations (50). Additionally, altered Treg responses occur 2 2 2 2 sical proinflammatory mediator. This view is supported by with adoptive transfer of C5ar1 / /C3ar1 / T cells into 2 2 2 2 2 2 evidence that C3ar1 / animals have modest pathology reduc- awild-typeanimal(47).C5ar1 / /C3ar1 / Tregs also dem- tions in chronic disease models, such as rheumatoid arthritis onstrate prolonged survival and enhanced function, suggesting (40). In contrast to the effects of C3a on purified monocytes that C3aR/C5aR1 stimulation of T cells is also important in The Journal of Immunology 3545 regulating inflammatory responses (51). C3aR signaling in determining the cause and effect in these studies is not possible APCs suppresses IL-4 production, inhibiting a Th2-polarized (56). The significant difficulty with research into the effects of response (52). C3aR signaling also acts in concert with C5aR1 the complement activation peptides is that their genesis also signaling to suppress the production of TGF-b1 from dendritic amplifies the terminal elements of the complement cascade. cells, reducing the stimulus for differentiation to Tregs and In the past, without the use of knockout mice or specific removing the inhibition on the Th1 response (47). However, in antagonists, it has been difficult to tease these aspects apart. vivo models of allergic asthma demonstrate that the type of C3-deficient animals have been used for investigation of experimental Ag used can alter the T cell response promoted by complement-mediated pathogen clearance during sepsis; it C3a, suggesting a complicated role for this molecule in T cell was demonstrated, perhaps unsurprisingly, that the lack of C3 biology (46, 53). reduces survival rates and increases pathogen load in experi- mental models (57). It might be expected that the increased C3a in disease pathogen load due to the loss of the terminal elements of the Ischemia-reperfusion injury. Intestinal ischemia-reperfusion (IR) explains the premature death of C3- injury, through transient occlusion of the mesenteric arterial tree, deficient animals. It remains to be seen whether any of the causes an increase in circulating neutrophils upon reperfusion direct antimicrobial actions of C3a are of sufficient magnitude that are critical to the generation and promulgation of tissue to contribute to this effect (13). injury (54). Tissue injury also directly activates the complement Attributing the effects of C3 deficiency to an attenuation of system, leading to the generation of the peptides C5a and C3a complement action is complicated by the fact that C3 is not Downloaded from (31). The role for C5a in the progression of this pathology is both an absolute prerequisite for the formation of the terminal ele- very well understood and unambiguous; C5a functions at the ments of the complement cascade, because C5 can be cleaved level of injury to cause extravasation and degranulation of the by serum proteases (10). These proteases are upregulated in 2 2 circulating neutrophils (55). By direct contrast, we demonstrated C3 / animals, leading to pathophysiologically significant in a model of intestinal IR that the role for C3a in this acute generation of C5a and C5b-9 (58). In keeping with this, pathology is overwhelmingly anti-inflammatory (32). Thus, the the differences demonstrated in bacterial load between C3- http://www.jimmunol.org/ historicalmodelforC3aasbeinga“weaker”formofC5aas deficient and C3-supplemented animals are not as dramatic as a proinflammatory mediator is no longer uncomplicated. the survival curves suggest (59). It is difficult to appreciate C3a levels in both blood and intestinal tissues increase after from this evidence whether it is the bactericidal or the C3aR- a period of ischemia, likely from activation of the complement mediated actions of C3 supplementation that contribute to system in response to damage-associated molecular patterns improved survival in septic animals. However, the initial re- 2 2 (32). Additionally, at the site of acute inflammation, car- port of a C3ar1 / mouse included an investigation into boxypeptidases preferentially degrade C5a over C3a, perhaps sepsis survival and concluded that C3aR signaling alone was

allowing for C3a to escape degradation and exert systemic key to improved survival (60). Additionally, this study dem- by guest on September 23, 2021 actions (8). We recently showed that one function of this C3a onstrated elevated circulating levels of the proinflammatory 2 2 release is to attenuate the neutrophilia associated with injury cytokine IL-1b in C3ar1 / animals. Further studies by the by confining unmobilized neutrophils to the bone marrow same group used heat-inactivated Escherichia coli administra- reservoir (32). Mice lacking C3aR have a significantly greater tion as a model of Gram-negative bacteremic sepsis. This increase in circulating neutrophils post-IR than do their wild- research also demonstrated a protective effect for C3aR, be- 2 2 type counterparts. This correlates with increased neutrophil cause C3ar1 / animals had significantly elevated mortality migration into ischemic tissue and worsened histopathological compared with their wild-type counterparts (61). Conversely, 2 2 outcomes. The effect of the C3a–C3aR axis in this acute injury C5ar1 / animals were protected from the bacteremia, again also appears to be restricted solely to an effect on neutrophils, demonstrating the divergent roles of these two molecules (61). because circulating levels of other leukocytes remained similar Although these investigators did not include measurements to wild-type animals. Additionally, the wild-type phenotypic of circulating neutrophils in the LPS- or E. coli–challenged response to IR could be fully restored through wild-type bone animals, recent studies surrounding the actions of C3a in 2 2 marrow transfer to C3ar1 / animals, indicating that the anti- inflammatory disease suggest that a reactive neutrophilia may 2 2 inflammatory actions of C3a take place at the level of the have been the cause of the reduced survival in C3ar1 / bone marrow rather than at the site of injury (32). Finally, animals. In support of this, the investigators noted that the 2 2 we showed a direct requirement for C3aR to restrict G-CSF– elevated IL-1b previously attributed to high C3ar1 / mor- mediated neutrophil mobilization from the bone marrow, in tality in the LPS model was unlikely to be the cause of death the absence of tissue injury (32). in bacteremia; instead, they suggested that the role of C3a is Because neutrophils are the major contributing cell type to regulate the inflammatory response generated through C5a responsible for propagation of the acute inflammatory re- signaling (60, 61). sponse, this emerging evidence of anti-inflammatory activity Renal disease. Several studies outlined the pathogenic role that for C3a through the retention of neutrophils in the bone complement plays in the development of autoimmune lupus marrow forms the crux of the thesis for this review: that C3a nephritis. High concentrations of C3aR and C3 are present should not be grouped with C5a as a purely proinflammatory in the glomeruli, correlating with disease severity (62), and anaphylatoxin. Indeed, several other in vivo studies in the seemingly suggesting a pathophysiological role for the C3a– literature demonstrate a more complex role for C3a in disease C3aR axis and, perhaps, a therapeutic target for treatment of models, particularly in the acute setting. lupus nephritis. It was demonstrated previously that infusion Sepsis. In sepsis, it was noted previously that elevated C3a and of the C3aR antagonist SB290157 in the MRL/lpr mouse C5a correlated with fatal outcomes in hospital patients, but model of lupus nephritis resulted in increased survival and 3546 BRIEF REVIEWS: C3a AS A PRO-/ANTI-INFLAMMATORY MOLECULE reduced renal inflammation (63). However, the same mouse grouped conceptually with its cousin, C5a. The reality is rather model backcrossed with C3aR-knockout mice indicates that more subtle and nuanced. It is true that C3a has several pro- the loss of C3aR accelerates renal injury and increases inflammatory facets, but in this review we highlighted nu- mortality (64). This contradictory result was attributed to merous studies from a wide variety of independent labora- the ill-defined pharmacological activity of the widely used tories that consistently point to significant anti-inflammatory C3aR antagonist, SB290157, which was reported to have functions for C3a, in direct functional opposition to C5a. It agonist activity upon binding C3aR (2). However, although also should be noted that different mediators that arise from the pharmacology of SB290157 may be controversial, the the same metabolic pathway, as do C3a and C5a, are often conclusion drawn by the investigators could be reinterpreted physiological antagonists in vivo. A good example is the pros- to take into account the acute “antineutrophil” effects of tanoids: individual mediators are well recognized to act on C3aR that were discovered recently (32). It is uncontroversial their cognate receptor and, with opposing effects, are physi- that, at least at the level of the nephron, C3aR is involved in the ological antagonists of each other. Therefore, we should not be pathogenesis of renal disease, including the induction of surprised by the concept that C3a and C5a may also function metaplasia in nephron epithelial cells central to the disease as physiological antagonists of each other. Indeed, C3a and (65, 66). These actions appear synergistic to the role of C5aR1 C5a have directly opposing effects on blood pressure mediated at the tissue level, but they may be redundant in the presence entirely through prostanoid activity (67). Collectively, the of elevated C5a concentrations, which is a more potent studies presented in this review contravene the current dogma proinflammatory mediator than C3a, at the receptor level. and suggest a change to the nomenclature or at least a wide- Downloaded from Contrary to reports of intestinal IR, there is a slight im- ranging recognition of the unforeseen complexity of the path- 2 2 provement in disease outcomes for C3ar1 / animals subjected ophysiological roles of C3a. At the very least, there should be to renal IR injury (65). In this study, the investigators dem- earnest attention and debate to leaven our understanding of onstrated that C3a administered directly to renal tubule epi- what is clearly a complex and multifaceted issue. thelial cells increased the production of chemotactic factors The response of C3a to injury is a dichotomy. In the acute directing leukocytes to the site of injury. Interestingly, a de- setting, C3a prevents mobilization of neutrophils, limits their http://www.jimmunol.org/ crease in global leukocytes was demonstrated in the diseased accumulation into tissues, and, therefore, reduces the inflam- 2 2 C3ar1 / kidney isolates, as indicated by CD45+ flow cy- matory response at the tissue level. However, in certain chronic tometry. However, there was no difference from wild-type mice disease models, such as asthma and rheumatoid arthritis, C3a with regard to granulocyte infiltration, suggesting that the dy- clearly demonstrates certain proinflammatory actions and namics of this model may be more complex than presented. contributes to disease progression (8). The difference in the Quantification of circulating granulocytes was not recorded in response of inflamed tissues to C3a, between the acute and 2 2 this model, but it is tempting to hypothesize that C3ar1 / chronic phases of inflammation, may well be due to the dif-

animals exhibited an injury-induced neutrophilia, as previously fering cell types involved (e.g., neutrophils versus monocyte/ by guest on September 23, 2021 reported (32), which is tempered by the decreased cytokine macrophages; Fig. 2). However, the different facets of C3a in production at the tissue level, as noted by these investigators. disease are not mutually exclusive but instead depend upon the balance between the pro- and anti-inflammatory effects of the Conclusions molecule. Even in models of disease that demonstrate a worse The terminology surrounding the effects of C3a in the liter- outcome with disruption of the C3a–C3aR axis, there are also ature warrants revisitation. C3a is frequently and consistently measurable benefits, such as reduced tissue proinflammatory described as proinflammatory in nature in the literature and is cytokines in the intestinal IR model (32). It is the balance of

FIGURE 3. The balance of C3a actions determines the disease phenotype. C3a induces both pro- and anti-in- flammatory effects that contribute to the disease phenotype. Anti-inflammatory effects of C3a dominate the acute phase of inflammation; targeting of C3aR with an agonist at this stage may result in beneficial effects on pathological out- comes. In contrast, the proinflammatory aspects of the molecule dominate the chronic phase of inflammation, and diseases characterized by their chronicity, such as rheuma- toid arthritis and asthma, may benefit from treatment with a C3aR antagonist. The Journal of Immunology 3547 these actions in disease that determines the ultimate outcome 15. Bellows-Peterson, M. L., H. K. Fung, C. A. Floudas, C. A. Kieslich, L. Zhang, D. Morikis, K. J. Wareham, P. N. Monk, O. A. Hawksworth, and T. M. Woodruff. (Fig. 3). 2012. De novo peptide design with agonist and antagonist activities: This dual role of C3a in inflammation also poses significant theoretical predictions and experimental validation. J. Med. Chem. 55: 4159–4168. opportunities for the development of therapeutics. It may be 16. Scully, C. C., J. S. Blakeney, R. Singh, H. N. Hoang, G. Abbenante, R. C. Reid, and D. P. Fairlie. 2010. Selective hexapeptide agonists and antagonists for human possible to harness the anti-inflammatory activity of C3a in complement C3a receptor. J. Med. Chem. 53: 4938–4948. the acute inflammatory phase through the use of C3aR ago- 17. Kalant, D., S. A. Cain, M. Maslowska, A. D. Sniderman, K. Cianflone, and P. N. Monk. 2003. The chemoattractant receptor-like protein C5L2 binds the C3a nists, of which there are emerging several promising candidates des-Arg77/acylation-stimulating protein. J. Biol. Chem. 278: 11123–11129. (68). We recently demonstrated the validity of this approach 18. Li, R., L. G. Coulthard, M. C. Wu, S. M. Taylor, and T. M. Woodruff. 2013. in an intestinal IR model (32). Equally, the proinflammatory C5L2: a controversial receptor of complement anaphylatoxin, C5a. FASEB J. 27: 855–864. aspects of C3a in chronic disease may warrant the targeting 19. Kalant, D., R. MacLaren, W. Cui, R. Samanta, P. N. Monk, S. A. Laporte, and of C3aR using selective antagonists to reduce the cytokine- K. Cianflone. 2005. C5L2 is a functional receptor for acylation-stimulating protein. J. Biol. Chem. 280: 23936–23944. mediated proinflammatory response. Future rational design of 20. Monsinjon, T., P. Gasque, P. Chan, A. Ischenko, J. J. Brady, and M. C. Fontaine. therapeutics targeting C3aR should take the dual roles of this 2003. Regulation by complement C3a and C5a of cytokine pro- duction in human umbilical vein endothelial cells. FASEB J. 17: 1003–1014. receptor into account. 21. Sayah, S., A. C. Jauneau, C. Patte, M. C. Tonon, H. Vaudry, and M. Fontaine. In this review we aimed to revisit the historical and recent 2003. Two different transduction pathways are activated by C3a and C5a ana- literature on the pathophysiological role of C3a in inflam- phylatoxins on astrocytes. Brain Res. Mol. Brain Res. 112: 53–60. 22. Shinjyo, N., A. Sta˚hlberg, M. Dragunow, M. Pekny, and M. Pekna. 2009. mation. In our view, it is time for a rethink and a careful, more Complement-derived anaphylatoxin C3a regulates in vitro differentiation and mi- nuanced consideration of the roles of C3a in inflammation, gration of neural progenitor cells. Stem Cells 27: 2824–2832. Downloaded from 23. Langkabel, P., J. Zwirner, and M. Oppermann. 1999. Ligand-induced phosphor- which seem far more catholic than those of its more famous ylation of anaphylatoxin receptors C3aR and C5aR is mediated by G protein- and celebrated cousin, C5a. We suggest that future investi- coupled receptor kinases. Eur. J. 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